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# Need help on c2 question arithmetic watch

1. (Original post by LilacFlorence)
You need to use the equation Sn =1/2n[2a+(n−1)d]
You know from part a and b that a=87 and d=-3 and you are told that n=k and Sn=0 so sub these numbers in and solve to find k

Is it right so far?, I feel like am doing it wrong

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2. (Original post by am99)

Is it right so far?, I feel like am doing it wrong

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Not quite - on the second line of that question you've done 87 +2 not x2, and it should be -3k not +3k
3. (Original post by am99)
...
Hi! Moved this to the Maths forum as it could be helpful for other maths students
4. (Original post by LilacFlorence)
Not quite - on the second line of that question you've done 87 +2 not x2, and it should be -3k not +3k

Like this? It still feels wrong lol

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5. (Original post by am99)

Like this? It still feels wrong lol

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You don't need to expand the brackets. As the two things multiplied together make 0, one of them must equal 0. 1/2k can't = 0 as that would make k 0 and that wouldn't work with what you were given (you can't add up the first 0 terms) therefore 177-3k=0 and then solve that to give k=59
6. (Original post by LilacFlorence)
You don't need to expand the brackets. As the two things multiplied together make 0, one of them must equal 0. 1/2k can't = 0 as that would make k 0 and that wouldn't work with what you were given (you can't add up the first 0 terms) therefore 177-3k=0 and then solve that to give k=59
Ohhhhhh, I didn't even see that, thank you !

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